Contrary to what is written in a number of places, the Vilna Gaon was not opposed to individuals accepting Sabbath early, or congregations holding early services, etc, and there is just too much Talmudic proof as to the legitimacy of accepting the Sabbath early. Many like to point to Ma’aseh Rav 115, which reports that the Gaon always prayed ma’ariv after tzeith so that he could connect the reading of the Sh’ma in its proper time with the ‘amida, even on the Sabbath, meaning that he himself would not pray ma’ariv early on the Sabbath, but in Ma’aseh Rav 117 and the P’ulath Sachir ad loc we then see the legitimacy of those who do accept the Sabbath early. In 117 we read that ma’ariv must precede the Sabbath qiddush, while the P’ulath Sachir clarifies: the official acceptance of the Sabbath, which is usually ma’ariv but not necessarily, must precede the qiddush because qiddush itself can not also be the act of accepting the Sabbath. Anybody who wants can be strict on himself like the Vilna Gaon, and anybody who wants can follow the ancient halacha, which as we pointed out before, was often necessary. See Orah Hayim 261:3, which is firmly based on the Talmudic sources and logic: in the days before clocks, the weather and other factors made it difficult and sometimes impossible to tell the time, so the Sabbath had to be accepted early out of doubt. The Torah was not given to the ministering angels, and we should not demand that any one be held to a higher standard than the previous generations.
As for the common opinion that plag is the earliest time one can accept the Sabbath, Maimonides himself has no explicit opinion on this regard, and even in the laws of Sh’vithath Asor, where he mentions the obligation to begin observing the afflictions of Yom Kippur while it is still Yom Kippur eve, he does not mention how early one can do so, although it makes sense that at least with regard to accepting the Sabbath plag should be the earliest time, because as we saw above, the sages and the rishonim equated ma’ariv with accepting the Sabbath, and the earliest entertainable time for that is plag, which can also be seen from the Vilna Gaon’s commentary on Orah Hayim.
Then there’s the notion that one can accept the Sabbath “two hours” early, or two hours before the time when the Sabbath comes in on its own. This can be found in the Bah to Orah Hayim 261-263, but this opinion only works within the framework of Rabbeinu Tam’s understanding of the z’manim, and means that one accept the Sabbath two hours before Rabbeinu Tam’s “final nightfall,” which for us is well over an hour after the actual sunset, meaning that one basically accepts the Sabbath in any event after our plag. One can not transfer this opinion to our understanding and say that there is an opinion allowing for accepting the Sabbath two hours before what we consider to be the time when the sabbath must start, sunset. However, in a place sufficiently far away from the equator, it is conceivable that plag will sometimes be two or more hours before sunset, but I am not aware of any Jewish community in such an extreme place.